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goto) within a program
based upon some condition. A loop is a sequence of instructions that will
be repeatedly executed based on some condition. In this chapter we will
explore some of the common decision structures in 80x86 assembly language.
if..then..else, case (switch),
while, for etc., into equivalent assembly language sequences.
This chapter also discusses techniques you can use to improve the performance
of these control structures. The sections below that have a "*"
prefix are essential. Those sections with a "o" discuss advanced
topics that you may want to put off for a while.if..then..else statement in Pascal:
IF (condition is true) THEN stmt1 ELSE stmt2 ;Assembly language, as usual, offers much more flexibility when dealing with conditional statements. Consider the following Pascal statement:
IF ((X<Y) and (Z > T)) or (A <> B) THEN stmt1;A "brute force" approach to converting this statement into assembly language might produce:
mov cl, 1 ;Assume true
mov ax, X
cmp ax, Y
jl IsTrue
mov cl, 0 ;This one's false
IsTrue: mov ax, Z
cmp ax, T
jg AndTrue
mov cl, 0 ;It's false now
AndTrue: mov al, A
cmp al, B
je OrFalse
mov cl, 1 ;Its true if A <> B
OrFalse: cmp cl, 1
jne SkipStmt1
<Code for stmt1 goes here>
SkipStmt1:
As you can see, it takes a considerable number of conditional statements
just to process the expression in the example above. This roughly corresponds
to the (equivalent) Pascal statements:
cl := true;
IF (X >= Y) then cl := false;
IF (Z <= T) then cl := false;
IF (A <> B) THEN cl := true;
IF (CL = true) then stmt1;
Now compare this with the following "improved" code:
mov ax, A
cmp ax, B
jne DoStmt
mov ax, X
cmp ax, Y
jnl SkipStmt
mov ax, Z
cmp ax, T
jng SkipStmt
DoStmt:
<Place code for Stmt1 here>
SkipStmt:
Two things should be apparent from the code sequences above: first, a single
conditional statement in Pascal may require several conditional jumps in
assembly language; second, organization of complex expressions in a conditional
sequence can affect the efficiency of the code. Therefore, care should be
exercised when dealing with conditional sequences in assembly language.
if..then..else
statements, case statements, and indirect jumps. The
following sections will describe these program structures, how to use them,
and how to write them in assembly language.if..then or
if..then..else statement. These two statements take the following
form shown below:The if..then statement is just a special case of the if..then..else
statement (with an empty ELSE block). Therefore, we'll only consider
the more general if..then..else form. The basic implementation
of an if..then..else statement in 80x86 assembly language looks
something like this:
{Sequence of statements to test some condition}
Jcc ElseCode
{Sequence of statements corresponding to the THEN block}
jmp EndOfIF
ElseCode:
{Sequence of statements corresponding to the ELSE block}
EndOfIF:
Note: Jcc represents some conditional jump instruction. IF (a=b) then c := d else b := b + 1;to assembly language, you could use the following 80x86 code:
mov ax, a
cmp ax, b
jne ElseBlk
mov ax, d
mov c, ax
jmp EndOfIf
ElseBlk:
inc b
EndOfIf:
For simple expressions like (A=B) generating the proper code
for an if..then..else statement is almost trivial. Should the
expression become more complex, the associated assembly language code complexity
increases as well. Consider the following if statement presented
earlier:
IF ((X > Y) and (Z < T)) or (A<>B) THEN C := D;When processing complex
if statements such as this one, you'll
find the conversion task easier if you break this if statement
into a sequence of three different if statements as follows:
IF (A<>B) THEN C := D IF (X > Y) THEN IF (Z < T) THEN C := D;This conversion comes from the following Pascal equivalences:
IF (expr1 AND expr2) THEN stmt;is equivalent to
IF (expr1) THEN IF (expr2) THEN stmt;and
IF (expr1 OR expr2) THEN stmt;is equivalent to
IF (expr1) THEN stmt; IF (expr2) THEN stmt;In assembly language, the former
if statement becomes:
mov ax, A
cmp ax, B
jne DoIF
mov ax, X
cmp ax, Y
jng EndOfIf
mov ax, Z
cmp ax, T
jnl EndOfIf
DoIf:
mov ax, D
mov C, ax
EndOfIF:
As you can probably tell, the code necessary to test a condition can easily
become more complex than the statements appearing in the else and
then blocks. Although it seems somewhat paradoxical that it may take more
effort to test a condition than to act upon the results of that condition,
it happens all the time. Therefore, you should be prepared for this situation.
if..then..else statements. An elegant implementation of
the example above is:
; IF ((X > Y) AND (Z < T)) OR (A <> B) THEN C := D;
; Implemented as:
; IF (A <> B) THEN GOTO DoIF;
mov ax, A
cmp ax, B
jne DoIF
; IF NOT (X > Y) THEN GOTO EndOfIF;
mov ax, X
cmp ax, Y
jng EndOfIf
; IF NOT (Z < T) THEN GOTO EndOfIF ;
mov ax, Z
cmp ax, T
jnl EndOfIf
; THEN Block:
DoIf: mov ax, D
mov C, ax
; End of IF statement
EndOfIF:
Admittedly, this appears to be going overboard for such a simple example.
The following would probably suffice:
; IF ((X > Y) AND (Z < T)) OR (A <> B) THEN C := D;
; Test the boolean expression:
mov ax, A
cmp ax, B
jne DoIF
mov ax, X
cmp ax, Y
jng EndOfIf
mov ax, Z
cmp ax, T
jnl EndOfIf
; THEN Block:
DoIf: mov ax, D
mov C, ax
; End of IF statement
EndOfIF:
However, as your if statements become complex, the density
(and quality) of your comments become more and more important.